The invention relates to reproduction of pages described by a page description language. More particularly, the invention relates to reproduction of traps which are described in such pages in the context of color transformations.
Full color printing is typically carried out by producing four or more printing plates, one for each of the color inks used on a printing press, typically Cyan, Magenta, Yellow and Black. The plates are configured to selectively apply the different colors to desired portions of the printing medium. To reproduce, for example, a Cyan circle on a square Magenta background, the Magenta plate must be configured such that no Magenta ink is applied in the intended area of the Cyan circle, i.e, the Magenta plate is defined as a square with a circular xe2x80x9cholexe2x80x9d corresponding to the location of the Cyan circle, as shown schematically in FIG. 1.
Unfortunately, a printing press is generally not capable of perfectly aligning the printed areas produced by each of the inking stations (e.g., plates). This mis-registration of color may leave highly visible white xe2x80x9csliversxe2x80x9d at the borders between adjacent colored areas. In the example described above, a thin crescent shaped area may be visible between the Cyan circle and the Magenta background, as shown schematically in FIG. 2.
A process called xe2x80x9ctrappingxe2x80x9d is typically applied to reduce the visual impact of the misregistrations described above. Referring to the example above, a thin line in the same color as the circle (i.e. Cyan) may be added around the circle, while the corresponding xe2x80x9cholexe2x80x9d in the Magenta plate is unchanged. This results in mixing of colors along a narrow transition area, a blue area in the above example, which is generally a much less noticeable artifact than the alternative white transition area between the two regions, assuming that the mis-registration area is narrower than (i.e., completely covered by) the overprinted area. The overprinting area in the above example, generally referred to as xe2x80x9ctrapxe2x80x9d, is illustrated schematically in FIG. 3.
It is often desired to proof the printed pages before initiating voluminous final printing. Proofing is typically performed on a color printer capable of producing copies quickly and inexpensively. Because the inks and dyes used on these printers are generally not the same as those used on the printing press, color transformations are applied to the colors on the page to make the resultant combinations of colors, i.e., Cyan, Magenta, Yellow and Black, produce a proof print with colors as similar as possible to the colors that would eventually be printed by the printing press.
In the above example, to render a reasonable color approximation for the pure Cyan to be produced by the printing press, the Cyan circle on the proof may be calculated to require a smaller percentage of the proof printer""s Cyan ink than the 100% to be used by the printing press, while small amounts of Magenta, Yellow and/or Black ink may be added to provide a correct approximation.
To produce a proof of the trap around the circle, the trap should be printed in an appropriate color. In the example above, if the Cyan color of the solid circle is to be used as the basis for the trap transformation, then a small amount of Magenta may be required in order to obliterate the larger amount of Magenta of the square. This way, the trap would not be visible in the proof, as shown schematically in FIG. 4. Alternatively, if the trap is not printed at all, the circle appears in the proof without a trap.
When using a continuous tone raster, it is possible to take the Cyan component from the overprinted trap, and the Magenta component from the underlying square, and to use that combination for the color transformation. This enables the blue color of the trapped area to be accurately represented.
However, a continuous tone raster is not always available. In many printing devices, colors are reproduced by screening whereby each pixel represents either xe2x80x98inkxe2x80x99 or xe2x80x98no inkxe2x80x99. Visually continuous tones may be emulated on such devices by varying the proportion of ink to no-ink pixels over a printed area. When the dots are sufficiently small, the printed area is perceived as a shade of color instead of a collection of individual dots.
Typically the pixels in a screened area are clumped together in screen dots. Darkening is achieved by adding more pixels to the clump. Brightening is achieved by deleting pixels from the clump, i.e., a brighter screen dot may be produced by a subset of the pixels of a corresponding darker dot. It is not necessary to store a continuous tone raster in order to produce the screened raster; in fact, it is advantageous not to store continuous tone rasters because their size requires additional processing.
Additionally, in certain applications, it may be desired to apply different color transformations to different parts of a page. For example, picture or image portions of a page may be treated differently from line-art portions of the page. Such differentiation on a continuous tone raster requires that each pixel be identified with a corresponding color transformation, thereby further increasing the size of continuous tone rasters. Therefore, it is advantageous to perform the color transformation before rasterizing the page.
Further still, certain types of proofing device require xe2x80x9cserialxe2x80x9d color delivery, whereby all of the ink or dye of a given colorant is delivered to the printing medium prior to the delivery of another colorant. Because color transformations require access to the tint values of all the inks or dyes, applying color transformations to a continuous tone raster requires simultaneous access to the tint values of all the colorants in order to produce a single resultant color. This scheme requires either a large amount of computer memory or repeated iterations over the entire stored a raster to produce each color.
It is noted that transforming the color of each pixel in an area of a given color is a tedious, memory consuming process. While there are known techniques for expediting the transformation of each pixel, such color transformations require significant memory usage. Further, if a continuous tone raster is not available for reference, when rasterizing a trap of a given shape and color, the color for the transformation cannot be synthesized.
In view of the above, there is a need for alternative, more efficient, methods for reproducing traps in printing of proofs.
An aspect of the present invention is based on the notion that the exact color of a trap is often not important, for example, it is rarely important in proof printing. While it is generally important to reasonably approximate the colors of traps, to properly remove visual artifacts on a proof, it is generally not important that the overlapping areas of the traps be printed in exact colors. For example, when the color transformation of pure Cyan (as in the above example) requires adding a small amount of Magenta and the trap overlaps a region printed with a larger amount of Magenta, it is generally unnecessary to reproduce the Magenta of the trap in the proof. However, when the color transformation requires adding a large amount of Magenta, for example, in regions where the color transformation results in a significant redistribution of colors (particularly Black versus the others colors) or where the background color is relatively faint, it is necessary to print the Magenta of the trap in order to produce an effective trap.
It is an object of the invention to provide a method of reproducing traps, e.g., in proof printing, wherein the colors of shapes are transformed independently of their background, assuming for the purposes of color transformation that the overprinted colors of traps would knock out. This results in a transformation of a trap in the same color as an adjacent solid color, e.g., the Cyan circle in the above example. However, in accordance with an aspect of the invention, the transformed component of an overprinting color is not reproduced if it is found to use less ink, i.e., to have a lower tint value, than the color to be overprinted (i.e., the color of the background) in the output raster.
According to one aspect of the invention, suitable for continuous tone rasters, when overprinting, the tint value of each colorant of each pixel in the calculated raster is compared with a corresponding transformed tint value, and the pixel is replaced only if its tint value is smaller than the transformed tint value. This is in contrast with painting models as are known in the art, wherein a new opaque color always replaces the old or wherein a shape is not rendered at all for the overprinted color.
According to another aspect of invention, suitable for use with halftone screened pages, the method utilizes the fact that a lighter screen dot may be formed from a proper subset of pixels of a corresponding darker screen dot. When rendering an overprinted area using a screen, the invention enables printing of the darker of the new color and the background by applying only the inked pixels of the screen (leaving the non-inked pixels as they were in the background), instead of applying both the inked and non-inked pixels of the screen to the raster. The method in accordance with this aspect of the invention is effective because, if the new color is lighter than the background color, all the inked dots applied to the overprinted area are inked anyway, forming part of the larger screen dot of the darker background, and if the new color is darker than the background, then the newly inked dots cover the background as well as additional dots.
Therefore, in both continuous tone and halftone renditions, for the overprinted color components, the resultant color of the new shape is a physical combination of the colorants of each of the transformed color representations, wherein the tint value for each colorant is the larger of the foreground and the background. This is not calorimetrically equivalent to transforming a combination of the color components of the new shape that knock out the background and the overprinted components from the knocked out background. However, it is a reasonable approximation which results in the reproduction of an effective trap on the proof. Further, this approximation enables the colors to be transformed without reference to the background, using screened backgrounds if required. The color transformation in accordance with the invention may vary from shape to shape. Additionally, according to the invention, the color transformation may be performed only once for each shape, rather than once for each pixel in the raster (as in the prior art).
When the overprinted area is a vignette, i.e., a region in which the color changes from light to dark or from one color to another, the method of the invention may include a further step of determining which colors should be overprinted with the transformed trap. As is known in the art, a vignette typically consists of a series of thin shapes (xe2x80x9cstepsxe2x80x9d), each step having a color slightly different from adjacent steps. According to the embodiment of the invention described above, only colorants that have a zero tint value before transformation are overprinted. However, unlike solid color regions, a vignette may include zero-value tinted steps adjacent to non-zero value tinted steps.
If each and every zero value tinted step in the vignette were to be overprinted using the technique described above (when color transformation is involved) or using conventional trap reproduction techniques (when color transformation is not applied), the background would show through only in one of the steps. To obtain an effective trap, however, the background should only show through vignettes that have a zero-value tint in each and every step. Therefore, once it is determined that a collection of steps forms a vignette, the set of color components to be overprinted (conventionally or using the technique of the invention described above) is determined by the conjunction of the sets of zero color components of all the steps in the vignette.
It is noted that certain vignettes are formed in a page description language by drawing successively smaller shapes one on top of another, wherein each shape completely encloses subsequent shapes. For such configurations, rendering of the vignette in accordance with the invention includes a reorganization whereby colorant values are compared between the vignette elements and the background, and not between successive vignette elements.